The following examples (or “constitutions”) as described below have been proposed as an optical compensation sheet for obtaining a wide viewing angle of a liquid crystal display:
(1) a method providing a discotic liquid crystal compound, which is a negative uniaxial compound, on a support;
(2) a method of providing on a support a nematic polymeric liquid crystal compound with a positive optical anisotropy, which is subjected to hybrid orientation in which the pretilt angle of the liquid crystal molecules varies in the thickness direction; and
(3) a method providing on a support two layers containing a nematic liquid crystal compound with a positive optical anisotropy, in which the orientation direction of the layers crosses each other at approximately 90 degrees, so that an optical property approximate to a negative uniaxial optical property is obtained.
However, the above examples (or “constitutions”) have the following problems.
Method (1) shows a defect specific to a discotic liquid crystal compound in that, in a TN mode liquid crystal display panel employing the discotic liquid crystal compound, the displayed image appears yellow when viewing the panel obliquely.
In method (1), the temperature developing the liquid crystal is high and therefore orientation cannot be fixed on an isotropic transparent support such as TAC (cellulose triacetate). This requires additional processing, in which a liquid crystal compound is oriented and fixed on a first support and transferred onto a second support such as TAC. Since this processing is more complex, it results in lower productivity.
There is disclosed in, for example, Japanese Patent O.P.I. Publication No. 8-15681, one example of an optically anisotropic layer employing a positive uniaxial low molecular weight liquid crystal compound according to method (3). The example is an optically anisotropic layer comprised of four layers which consist of a first oriented layer having an orientation ability, a rod-shaped, positive uniaxial low molecular weight liquid crystal compound layer, in which the liquid crystal compound is oriented and fixed, provided on the first oriented layer, a second oriented layer having an orientation ability provided on the rod-shaped, positive uniaxial low molecular weight liquid crystal compound layer, and a rod-shaped, positive uniaxial low molecular weight liquid crystal compound layer, in which the liquid crystal compound is oriented and fixed, provided on the second oriented layer. In this example, a property approximate to a disc-shaped compound can be obtained, for example, by arranging the two rod-shaped, liquid crystal compound layers so that the orientation directions in the plane of the two layers cross each other at 90 degrees.
Accordingly, method (3) above is extremely advantageous for use in a liquid crystal TV (television) giving priority to color reproduction, since there is no problem related to a yellowing occurring with the use of a discotic liquid crystal compound.
Although the use of the discotic liquid crystal compound requires only one layer, method (3) requires two liquid crystal compound layers, which results in lower efficiency.
However, the above three methods have a common fundamental problem. They require an optical compensation sheet to be provided on both sides of, for example, a liquid crystal cell in order to achieve optical compensation, resulting in a cost increase. Further, in these methods, the use of one optical compensation sheet destroys symmetry, and results in asymmetry of the viewing angle. For example, when the optical compensation sheet is arranged so that the rubbing axis is rotated 45 degrees, symmetry may be improved but the viewing angle property is not improved. There have been no proposals in which the use of only one, optical compensation sheet improves the viewing angle property to the same degree as or more than the use of two optical compensation sheets;